[Numpy-discussion] Should bool_ subclass int?

Alan G Isaac aisaac@american....
Tue Jul 10 09:36:55 CDT 2007


I found Gael's presentation rather puzzling for two reasons.

1. It appears to contain a `+` vs. `*` confusion.
See http://en.wikipedia.org/wiki/Two-element_Boolean_algebra

2. MUCH more importantly:
In implementations of TWO, we interpret `-` as unary 
complementation (not e.g. as additive inverse; note True 
does not have one).  So

    -True is False
    -False is True

This matches numpy:
>>> -N.array([False])
array([True], dtype=bool)
>>> -N.array([True])
array([False], dtype=bool) 

This is a GOOD THING.
However, a-b should then just be shorthand for a+(-b).
Here numpy does not in my opinion behave correctly:

>>> N.array([False])-N.array([True])
array([True], dtype=bool)
>>> N.array([False])+(-N.array([True]))
array([False], dtype=bool)

The second answer is the right one, in this context.
I would call this second answer a bug.

Cheers,
Alan Isaac





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